ueberauth Guardian EUVD-2026-51915

Exploitation requires the target application to pass an attacker-influenced binary as the key argument to any of the following functions: Guardian.Plug.current_token/2, Guardian.Plug.current_claims/2, Guardian.Plug.current_resource/2, or directly to Guardian.Plug.Keys.base_key/1, token_key/1, claims_key/1, or resource_key/1. The attacker-influenced value must vary across requests - sending the same key repeatedly has no effect because atoms are only interned once. Applications that use exclusively static, compile-time atom literals for the key parameter (the idiomatic default, e.g., key: :default) are not exploitable. No authentication is required: an unauthenticated HTTP endpoint is sufficient provided it routes request data into a Guardian key. The CVSS 4.0 AT:P metric confirms a specific application-level configuration prerequisite must be present; default Guardian deployments using only literal keys are not affected.

The provided CVSS 4.0 score of 6.9 with vector AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H assigns a local attack vector, which is somewhat conservative relative to the described exploitation path: the attack is delivered via network HTTP requests, not local access. The AT:P (Attack Requirements: Present) correctly captures the prerequisite - the application must route user-controlled data into a Guardian key parameter. The VA:H and SA:H metrics reflect the full-node crash impact accurately, since BEAM node termination denies service to all co-hosted applications simultaneously. No EPSS data was provided. There is no CISA KEV listing and no confirmed public exploit code, which lowers urgency for environments where user input never reaches Guardian key functions. However, multi-tenant Phoenix applications or any service using dynamic key selection are genuinely at risk: the attack is not gated by authentication (PR:N), requires no special tooling beyond an HTTP client, and produces a full, unrecoverable node crash rather than a degraded-service condition. The discrepancy between AV:L in the provided vector and the network-delivered attack path described in the advisory warrants verification with the vendor, though AT:P constrains real-world exploitability regardless of attack vector classification.

An attacker targeting a multi-tenant Phoenix application that derives the Guardian key from the HTTP request's tenant subdomain or X-Tenant-ID header sends a rapid stream of HTTP GET requests, each carrying a unique, previously unseen tenant string. Each distinct request causes Guardian.Plug.Keys.base_key/1 to call String.to_atom/1, permanently interning one new atom. After exhausting the remaining atom table capacity - which may require far fewer than 1,048,576 requests depending on existing atom usage - the BEAM runtime raises an out-of-atoms error, the node process terminates, and every application sharing that VM instance becomes immediately unavailable. No exploit code is required beyond a standard HTTP load tool with randomized header values.